1. Field of the Invention
This invention relates to safety pressure relief devices utilizing bursting discs, and more particularly to such devices utilizing fragmentable bursting discs in combination with safety relief valves.
2. Prior Art
Conventionally, a variety of arrangements are employed for safety pressure relief in pressurized systems such as pressurized conduits, storage or reaction tanks or other vessels. One arrangement involves use of a pressure relief or bursting disc. A bursting disc ruptures upon exposure to pressure in excess of a selected design pressure, thereby creating an opening in the pressurized system and allowing pressure relief.
Two basic types of bursting discs are generally employed for such safety pressure relief. A graphite disc or a simple or tension disc will fragment upon rupture of the disc. A reverse buckling disc does not fragment upon rupture of the disc. Instead, under pressure exceeding a predetermined limit, a reverse buckling disc is pressed against cutting blades, causing the disc to be sliced; and the pressurized fluid forces the disc slices to "peel" back or retract out of the path of the fluid.
Use of bursting discs, however, involves several drawbacks. For example, bursting discs are not reusable. Rupture of a bursting disc destroys the disc, necessitating its replacement. In addition, since a bursting disc does not automatically reseal itself (or reseal itself at all for that matter) after rupture, the contents of a vessel protected by a bursting disc tend to flow out of the opening created by the ruptured disc and to continue to flow out of the opening until the pressure inside the vessel equalizes with the pressure outside the system. Thus, large quantities of the system contents may be lost; and further, contamination of the system surroundings by the system contents may create dangerous conditions.
Another method for safety pressure relief, use of a pressure relief valve, may avoid such drawbacks. However, use of safety relief valves also entails attendant disadvantages. For example, exposure of the pressure relief valve to the contents of the vessel or conduit, as typically occurs in attachment of the valve directly to the system, may result in corrosion of the valve. In an effort to avoid corrosion, valves formed of expensive materials such as stainless steel are often employed, increasing system costs. Moreover, the vessel or conduit contents may build up on the internal surfaces of the valve, blocking or plugging up the valve. Development of such corrosion or buildup tends to interfere with or to prevent proper operation of the valve.
In an effort to alleviate the drawbacks of bursting discs, as well as those of safety relief valves, bursting discs have been installed in combination with pressure relief valves. According to this method, a bursting disc is installed in a closed system between a pressure relief valve and the vessel or conduit. The contents of the vessel or conduit are thus isolated from the pressure relief valve, thereby avoiding corrosion of or buildup on the valve. Moreover, when the bursting disc ruptures, the safety relief valve will generally release the contents of the vessel or conduit until the pressure in the vessel or conduit falls below the design pressure of the valve. At that point, the valve reseals, retaining the remaining vessel or conduit contents within the vessel or conduit.
However, even this method has been found to suffer serious drawbacks. When a fragmentable bursting disc is used, upon rupture of the disc, fragments from the disc tend to be projected at high velocity toward the pressure relief valve, thereby damaging the valve or becoming so situated as to disrupt an effective seating of the valve and preventing resealing of the vessel or conduit. On the other hand, use of non-fragmentable bursting discs is undesirable for several reasons. Such discs not only are fragile, but also are more expensive than fragmentable bursting discs. Further, since the relatively incompressible nature of liquids in liquid filled systems allows pressure relief upon release of merely a small amount of liquids, non-fragmentable bursting discs tend not to rupture completely and so are not generally acceptable for liquid filled systems.